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Abstract:

An electrospinning method includes providing a nozzle fluidically coupled
to a source of polymer ink and providing a substrate adjacent to the
nozzle. A first voltage is applied to the nozzle to initiate
electrospinning of the polymer ink onto the substrate, wherein the first
voltage is within the range of about 400V to about 1000V. The voltage is
then reduced to a second, lower voltage wherein the voltage is within the
range of about 600V to about 150V.

Claims:

1. An electrospinning method comprising: providing a nozzle fluidically
coupled to a source of polymer ink; providing a substrate adjacent to the
nozzle; applying a first voltage to the nozzle to initiate
electrospinning of the polymer ink onto the substrate, wherein the first
voltage is within the range of about 400V to about 1000V; reducing the
voltage to a second, lower voltage wherein the voltage is within the
range of about 600V to about 150V.

2. The method of claim 1, further comprising moving the substrate in at
least one of the x, y, and z directions relative to the nozzle.

3. The method of claim 1, further comprising moving the nozzle in at
least one of the x, y, and z directions relative to the substrate.

4. The method of claim 2, further comprising controlling at least one of
the acceleration, deceleration, or speed of the substrate to cause a
gradual change in the thickness of the electrospun ink.

5. The method of claim 3, further comprising controlling at least one of
the acceleration, deceleration, or speed of the nozzle to cause a gradual
change in the thickness of the electrospun ink.

6. The method of claim 1, wherein the distance between the nozzle and the
substrate is within the range of about 1 mm and about 3 mm.

7. The method of claim 2, wherein substrate moves relative to the nozzle
such that the electrospun polymer can be further stretched mechanically.

8. The method of claim 1, further comprising pyrolysing the polymer ink.

9. The method of claim 1, wherein the polymer ink comprises high
molecular weight PEO with an aqueous dispersion of PEDOT:PSS.

14. An electrospinning device comprising: a moveable stage configured to
hold a substrate; an electrode nozzle disposed at a distance from the
moveable stage; a power source operatively coupled to the electrode
nozzle and the substrate; a controller operatively coupled to the
moveable stage and the power source, the controller controlling the
relative speed between the moveable stage and the electrode nozzle as
well as an applied voltage to the nozzle by the power source.

15. The electrospinning device of claim 14, wherein the controller is
configured to initially apply a first voltage within the range of about
400V to about 1000V and subsequently apply a second voltage within the
range of about 600V to about 150V.

16. The electrospinning device of claim 15, wherein the controller is
configured to move the moveable stage such that the electrospun polymer
can be further stretched mechanically.

17. The electrospinning device of claim 14, wherein electrode nozzle is
located between about 1 mm and 3 mm from the moveable stage.

18. The electrospinning device of claim 14, further comprising a pump
operatively coupled to the electrode nozzle.

19. The electrospinning device of claim 14, wherein the substrate is a
planar substrate.

20. The electrospinning device of claim 14, wherein the substrate is
three dimensional.

Description:

RELATED APPLICATION

[0001] This Application claims priority to U.S. Provisional Patent
Application No. 61/466,871, filed on Mar. 23, 2011, which is hereby
incorporated by reference in its entirety. Priority is claimed pursuant
to 35 U.S.C. §119.

FIELD OF THE INVENTION

[0003] The present invention pertains to methods that use low-voltage,
near-field electrospinning to allow for the controlled and continuous
electrospinning of nanofibers. The electrospinning system uses a
superelastic polymer ink at low voltage so that the nanofibers may be
controlled and patterned.

BACKGROUND

[0004] Fabrication of polymeric nanofibers may be used in a wide variety
of applications such as in the fields of sensors and actuators, energy
storage, smart textiles, optoelectronics, tissue engineering, medical
device fabrication, prosthetics, drug delivery, microresonators, and
piezoelectric energy generators. Several processes have been developed to
tailor the properties of polymeric nanofibers to suit the particular
needs of each application. These polymeric nanofiber modification
techniques include chemical modification, surface deposition of metals,
functional doping, and composite formation. Polymeric nanofibers can also
be pyrolyzed to yield thinner carbon nanofibers, opening up an even wider
range of applications, including electrochemical sensors and energy
storage.

[0005] Polymeric nanofibers may be useful as diodes. The Schottky diode is
a semiconductor diode with a low forward voltage drop and a fast
switching action. When current flows through a diode there is a small
voltage drop across the diode terminals. A normal silicon diode has a
voltage drop between 0.6-1.7 volts, while a Schottky diode voltage drop
is between approximately 0.15-0.45 volts. This lower voltage drop can
provide higher switching speed and better system efficiency.

[0006] To form a Schottky diode, a metal-semiconductor junction is formed
between a metal and a semiconductor, creating a Schottky barrier instead
of a semiconductor-semiconductor junction as in conventional diodes.
Typical metals used are molybdenum, platinum, chromium or tungsten; and
the semiconductor would typically be N-type silicon. The metal side acts
as the anode and N-type semiconductor acts as the cathode of the diode.
This Schottky barrier results in both fast switching and low forward
voltage drop.

[0007] One of the key factors in the utilization of polymeric nanofibers
in many of the aforementioned applications is the ability to accurately
control the physical properties and positioning (patterning) of the
produced nanofibers. One option for continuous patterning of polymer
nanofibers is far-field electrospinning (FFES), which is a well-known
technique to produce polymeric nanofiber mats in large quantities.
Conventional Far-Field Electrospinning (FFES) involves application of 10
to 15 kV to propel a polymer jet from a biased syringe nozzle towards a
grounded substrate electrode. Typically in FFES, the syringe-to-substrate
distance is in the range of several centimeters, e.g., around 10-15 cm.
Unfortunately, the high voltage used in FFES causes bending instabilities
in the jet that leads to chaotic whipping motion of the depositing
nanofibers. This whipping motion makes it difficult to control the
position of where the nanofibers land on the substrate.

[0008] Although work has been carried out to achieve alignment of
nanofibers along a prescribed direction through the use of a rotating
drum collector, and by using electrical field manipulation, precise 2D
and 3D patterning is still very difficult to achieve with FFES.

[0009] Recent efforts on a variant of electrospinning called near-field
electrospinnning (NFES) produced some encouraging initial results,
opening up a possibility of achieving scalable precision patterning with
polymeric nanofibers. NFES offers the advantage of large scale
manufacturability (inherent in electrospinning) combined with controlled
electric field guidance (due to a reduced distance between the source and
collector electrodes). However, the reported efforts required the use of
electric fields well in excess of 200 kV/m for continuous NFES operation
so that the resulting polymer jets still exhibit bending instabilities
and thus limited control of polymeric nanofiber patterning. For example,
Chang et al. disclose continuous near-field electrospinning for large
area deposition of orderly nanofiber patterns using an electric field of
at least 1,200 kV/m (applied voltage of 600V to syringe needle). See
Chang et al., Continuous Near-Field Electrospinning For Large Area
Deposition of Orderly Nanofiber Patters, Appl. Phys. Lett. 93, 123111
(2008).

SUMMARY

[0010] In one embodiment, an electrospinning method includes providing a
nozzle fluidically coupled to a source of polymer ink and providing a
substrate adjacent to the nozzle. A first voltage is applied to the
nozzle to initiate electrospinning of the polymer ink onto the substrate,
wherein the first voltage is within the range of about 400V to about
1000V. The voltage is then reduced to a second, lower voltage wherein the
voltage is within the range of 150V to about 600V.

[0011] In another embodiment, an electrospinning device includes a
moveable stage configured to hold a substrate; an electrode nozzle
disposed at a distance from the moveable stage; a power source
operatively coupled to the electrode nozzle and the substrate; a
controller operatively coupled to the moveable stage and the power
source, the controller controlling the relative speed between the
moveable stage and the electrode nozzle as well as an applied voltage to
the nozzle by the power source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1A is a schematic illustration of the typical components of a
NFES system.

[0013] FIG. 2 is a schematic illustration of additional components of a
NFES system.

[0014] FIGS. 3A and 3B show the deposition pattern of the polymer jet when
the applied voltage is 600 V and the nanofibers are formed from a polymer
ink of 2% wt PEO in an aqueous solution. The snakelike pattern is
believed to occur due to high-speed oscillatory bending instability in
the jet. Deposition is done at a stage speed (linear) of 10-40 mm/s.

[0015] FIGS. 3C and 3D show the deposition pattern of the polymer jet when
the voltage is 300V and the nanofibers are formed from a polymer ink of
2% wt PEO in an aqueous solution. Deposition is done at a stage speed
(linear) of 10-40 mm/s.

[0016] FIG. 4A illustrates a graph showing the diameter of the nanofiber
(i.e., nanofiber thickness) as a function of voltage applied between the
nozzle and the substrate.

[0017] FIG. 4B is a scanning electron microscope (SEM) image of a
continuously electrospun nanofiber with an abrupt change in voltage which
corresponds to a voltage reduction from 300V to 200V.

[0018] FIG. 5A illustrates a graph showing the diameter of the nanofiber
(i.e., nanofiber thickness) as a function of stage speed. The same
pattern has been set for all the samples, while the maximum speed varies
(applied voltage: 400V).

[0019] FIG. 5B illustrates a SEM image of aligned nanofibers continuously
electrospun according to the programmed pattern. Fiber thickness is shown
to depend on the velocity of X-Y stage. As seen in FIG. 5B, a slower
stage speed results in a thicker fiber while a faster stage speed results
in a thinner fiber.

[0020] FIG. 6 is a SEM image of a nanofiber patterned directly with low
voltage NFES at 200V. The fiber was coated with 6 nm Pd/Au to improve SEM
resolution.

[0021] FIG. 7 is a SEM image of multiple nanofibers suspended on CMP
arrays deposited by continuous NFES of viscoelastic 2 wt % PEO polymer at
300V. Six (6) posts are connected to each other by nanofibers.

[0022] FIG. 8 illustrates a schematic of the deposition layout of the
PEO:PEDOT:PSS nanofibers between the gold electrodes. The inset
schematically shows the arrangement of PEDOT:PSS islands in PEO solution.

[0023] FIG. 9A illustrate SEM images of PEDOT: PSS: PEO aligned nanofiber
arrays deposited between gold pads, one end of which is illustrated in
FIG. 9A.

[0031] FIG. 14 illustrates the degree of shrinkage in the diameter (i.e.,
thickness) of electrospun fibers after pyrolysis.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0032]FIG. 1A shows a typical NFES system 200. The system 200 includes a
dispensing electrode nozzle 201. A polymer droplet 202 is illustrated at
the end of the dispensing electrode nozzle 201. A Taylor Cone 203 is
generated near the polymer droplet 202 and a polymer jet is stretched by
the electric field whereby the polymer contacts the substrate 204. As
explained below, the substrate 204 may be a two dimensional substrate
(e.g., wafer) or in other embodiments, the substrate 204 is a
three-dimensional substrate (or a two-dimensional substrate with
three-dimensional features formed or disposed thereon). A high voltage
power supply 205 is coupled to the dispensing electrode nozzle 201 and
the substrate 204 with the substrate 204 acting as ground. The distance
from the dispensing electrode nozzle 201 to substrate 204 can be adjusted
using an x-y-z motion stage 303 as seen in FIG. 2. In an alternative
embodiment, the stage that holds the substrate 204 may be stationary and
the dispensing electrode nozzle 201 is moveable in at least one of the x,
y, and z directions. The NFES system 200 may also include an optional
computer, 301, as shown in FIG. 2, a microscope or camera 302 to record
or observe the nanofibers, and an x-y-z motion stage 303 on which a
substrate 304 is mounted to collect the nanofibers. The NFES system 200
further includes a power supply 205 that applies the voltage to the
dispensing electrode nozzle 201. The computer 301 may also be used to
control the power supply 205. A pump 306 such as a syringe pump is loaded
with polymer and (or a container fluidically coupled to the pump is
loaded with polymer) is activated to provide a continuous source of
polymer to the dispensing electrode nozzle 201.

[0033] The computer 301 may include software stored therein (e.g., LabView
or some other software) that is used control various aspects of the
system. For example, the computer 301 may control the voltage levels
(timing of their application to the nozzle 201) that are applied to the
dispensing electrode nozzle 201. The computer 301 may also control other
components of the system like the motion stage 303 (e.g., patterns,
speed, acceleration, deceleration, and distance between nozzle and
substrate). The computer 301 may also control the pump 306. Image
acquisition and data analysis, if needed, can also be implemented using
the computer 301.

[0034] FIG. 1B illustrates a block diagram for the control architecture,
according to one embodiment, for implementing the method of low voltage
NFES (LV-NFES). As seen in FIG. 1B, the computer 301 interfaces with
camera 302. The computer 301 also interfaces with a servo controller 350
(Phidgets 1061 Advanced Servo 8-motor controller) that is used to control
a linear actuator 352 and humidifier 354 (via humidifier servo 356). The
linear actuator 352 is used disrupt the polymer droplet on the nozzle 201
with a sharp tungsten or glass tip. The humidifier 354 controls the
relatively humidity surrounding the device. A relatively humidity of
around 60% permits the formation of stable and continuous patterns. A
humidifier 354 with a feedback control from a humidity sensor 358 (via
interface 359) such as the Phidgets 1125 Humidity/Temperature sensor may
be used and maintains a relative humidity within +/-3%. The computer 301
also interfaces with an pneumatic pump 306 that that is connected to a
syringe 308 that dispenses the polymer ink. The computer 301 interfaces
with the x-y-z stage 204 via stage controller 360.

[0035] In one embodiment, a LV-NFES method is initiated with a first or
initiation voltage between the range of about 1000V to about 400V and
then the voltage is dropped to a second, operating voltage as low as
200V. For example, the second, operating voltage may be within the range
of about 600V to about 150V. The method uses a superelastic polymer
solution pumped through a nozzle 201 (e.g., needle) to allow for
continuous and controlled electrospinning of polymeric nanofibers. The
operating distance between the nozzle 201 and the substrate 304 for this
NFES set-up may adjustable by is approximately 1 mm. In some instances,
the distance is between about 1 mm and several mm (e.g., 3 mm) This
method is intended to address the problem of bending instabilities caused
by the high voltage used in NFES. By using a lower voltage, the bending
instabilities of the polymer jet are reduced and better control of the
polymer jet is enabled allowing for better positioning of the resulting
nanofiber formed by LV-NFES.

[0036] A superelastic polymer solution is one that can be stretched to
enormous strains without breaking. Solutions of such superelastic
polymers contain long entangled polymer chains that promote
stretchability and are expected to augment continuity of the electrospun
jets. This facilitates the continuous electrospinning of the polymer jet
into nanofibers. Nanofibers produced at a voltage of 600V with a
superelastic polymer such as polyethylene oxide (PEO) in a 2% wt solution
in deionized water are shown in FIGS. 3A and 3B. FIGS. 3A and 3B
illustrate the looped nanofibers caused by the high-speed oscillatory
bending instability of the polymer jet. The same solution of PEO 2% wt in
deionized water produces straight, aligned nanofibers at 300V as shown in
FIGS. 4C and 4D without the looped nanofibers at 600V. The lower voltage
of 300V minimizes the bending instabilities of the polymer jet so that
nanofibers may be controlled and aligned.

[0037] The diameter of the nanofibers may also be varied by changing the
voltage. In FIG. 4A, a graph of nanofiber diameter as a function of
voltage is presented. As seen in FIG. 4A, at higher voltages, the
nanofiber is thicker and with lower voltage, the nanofiber is thinner.
The SEM image of a nanofiber in FIG. 4B shows the nanofiber's reduction
in diameter as the voltage was changed from 300V to 200V. A noticeable
decrease can be seen. In FIG. 4B a thicker nanofiber 501 is formed at
300V and a thinner fiber 502 is firmed at 200V.

[0038] In another embodiment, the method may be used to produce a
nanofiber that is deposited on a substrate 304 and then a mechanical
force caused by the movement of an x-y-z stage 303 may pull the nanofiber
to thin the fiber. FIG. 5A illustrates a graph of the speed of the x-y-z
stage as a function of the diameter of the nanofiber. As seen in FIG. 5A,
the faster the x-y-z stage moves, the thinner the nanofiber. FIG. 5B
shows the SEM images of the thickness of nanofibers produced by varying
speeds of the x-y-z stage. FIG. 6 illustrates an SEM image of a 16.2 nm
nanofiber produced by the x-y-z stage moving at a speed of 100 mms-1
away from the nozzle such that the polymer jet is stretched (in the x-y
plane) at a nozzle voltage of 200V. In one embodiment, the mechanical
force may be caused by the x, y, or z movement of the x-y-z stage 303.
This movement may position the generated nanofiber onto a different
substrate or structure located on the same substrate. As an example, FIG.
7 illustrates multiple nanofibers 801 suspended on CMP arrays deposited
by continuous NFES of viscoelastic 2 wt % PEO polymer at an applied
voltage of 300V. Six (6) posts 802 are connected to each other by
nanofibers 801.

[0039] In one aspect of the invention, an electrospinning ink can be
formed by combining a conducting polymer with a superelastic polymer
solution to form electrospinning ink. As one example, a conducting
polymer such as Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) may be combined with the superelastic polymer solution. The
electrospinning ink may be prepared by mixing high molecular weight PEO
with an aqueous dispersion of the conducting polymer PEDOT:PSS using a
magnetic stir bar over an extended period of time (e.g., overnight).

[0040] In another aspect of the invention, an electrospinning ink can be
made by combining PEO with a carbonizable negative photoresist such as
SU8. SU8 can be pyrolysed into monolithic carbon structures after they
have been crosslinked by UV exposure. See C. Wang, G. Jia, L. Taherabadi,
and M. Madou, "A novel method for the fabrication of high-aspect ratio
C-MEMS structures," Microelectromechanical Systems, Journal of vol. 14,
no. 2, 2005, pp. 348-358, which is incorporated by reference herein.

[0041] The following are working examples of LV-NFES.

[0042] PEO Polymer Solution

[0043] High molecular weight polyethylene oxide (MW=4000000) from Dow Inc.
(WSR-301) was tested as the superelastic polymer ink at 1, 2, and 3 wt %,
respectively, in deionized (DI) water. To obtain homogeneous PEO
solutions, the PEO and the DI water were allowed to freely diffuse for 24
h followed by 96 h of vortex mixing in a single stirrer turbine at 30
rpm.

[0044] The low-voltage NFES experimental set-up used a 3 mL syringe bore
fitted with a 27 gauge (200 μm i.d.) type 304 stainless steel needle
as the nozzle 201 and was mounted on a syringe pump (Harvard Apparatus,
PHD 70-2001) to dispense the superelastic polymer ink at a feed rate
lower than 1 μL/h. Pyrolyzed SU 8 carbon and Si were used as
substrates. The voltage was applied to the stainless steel needle, while
the substrate was grounded. The substrate to needle distance was
maintained at 1 mm. The voltage was turned on after the polymer formed a
full-sized droplet of approximately 500 μm diameter at the needle tip,
held in place by surface tension. The polymer jet does not self-initiate
under the influence of the voltage because the electrostatic force cannot
overcome the surface tension at the droplet-air interface. Therefore, the
electrospinning process was initiated by introducing an artificial
instability at the droplet-air interface with a glass microprobe tip (1
to 3 μm tip diameter) that resulted in a very high local electric
field, sufficient to overcome the interfacial surface tension, giving
rise to the formation of the Taylor cone and initiation of the polymer
jet.

[0045] The patterning of nanofibers onto the substrate was carried out for
up to 45 min to produce a stable and controllable and continuous jet
using the low voltage method described herein. Among the concentrations
of PEO solutions that were tested, the use of about 2 wt % PEO solution
resulted in the most controlled continuous electrospinning. The lower
concentration at 1 wt % PEO formed a very thin electro-spinning jet that
pinched off easily within a few seconds of initiation. Possible reasons
for the latter are a faster loss of entanglement due to a lower
relaxation time and a lower viscosity that reduces jet resistance to the
bending instabilities, both causing easier breakage of the jet.
Conversely, the 3 wt % PEO solution forms a thicker jet due to its higher
viscosity and higher conductivity, both of which are known to increase
the effective polymer flow rate. The 3 wt % PEO jet tends to harden
before the onset of electrospinning and this hardening is likely caused
by premature solvent evaporation during its longer flight in air due to
an increased resistance to momentum change emanating from a higher
viscosity.

[0046] The polymer jet was initiated at a higher voltage within the range
of 400-600V at a first voltage level, also known as the initiation
voltage, to obtain a visible jet. After initiation the voltage was
lowered to a second, lower voltage level, i.e., an operational voltage,
which can be as low as about 200V with approximately 1 mm
source-to-substrate operating distance using 2% PEO. The operational
voltage at the second, lower level may fall within a lower range that
depends on the exact composition of the polymer. For example, PEO blended
with other polymers like PEDOT may have a higher "lower range" while
blending with high viscosity SU8 may lead to lower "lower range." It is
generally believed that the lower range of the second, lower level
voltage that will encompass most if not all such compositions is between
about 100V to about 300V. This is a significant improvement over
conventional FFES methods that utilize voltages in excess of 1,000V at
10-15 cm operating distances. The low-voltage NFES setup allows seamless
electrospinning with superior control of nanofiber thickness and
alignment.

[0048] Another advantage of lower voltage operation lies in reduction of
the diameter of the jet, leading to thinner nanofibers. This is most
likely due to the lower electrostatic forces at play that reduce the feed
rate of the polymer, thus reducing jet thickness. Therefore, the voltage
can be manipulated to directly control the thickness of the nanofibers.
Direct evidence of this relationship was observed in real time during
electrospinning when a stepwise reduction in voltage reduced the
thickness of the deposited nanofiber thus causing it to scatter less
light making it difficult to observe, as the voltage was reduced. The
deposited pattern went from a visible line at 400 V to almost invisible
at 200 V under 60× magnification in the stereo microscope used to
monitor the electrospinning process.

[0049] Low voltage operation at around 200V permits the patterning of very
thin nanofibers having diameters below 20 nm. Such ultrathin nanofibers
seem to be porous, perhaps an effect either due to beading of the
nanofibers or Pd/Au particle growth during sputtering. The fibers were
sputtered with 6 nm Pd/Au layer to improve SEM resolution. This method is
thus able to reproducibly pattern ultra-thin nanofibers in the range of
10-20 nm which cannot be accomplished using conventional far-field and
near-field electrospinning.

[0050] All experiments were conducted on an automated X-Y microstage
(Prior Scientific Inc.) that is programmed to move the substrate in any
desired pattern, for instance, in a perpendicular square wave pattern.
The speed of the X-Y stage has a significant effect on the physical
characteristics of the deposited nanofibers. As the stage accelerated to
reach a certain speed, or decelerated to change direction, the diameter
of the nanofiber was found to vary substantially. Generally, lower
average velocity leads to fiber thickening, and vice versa for a higher
average velocity, most likely resulting from the mechanical stretching of
the nanofibers between the point of contact on the substrate and the
droplet. While this effect can be avoided by patterning only in the
constant velocity regime, it is also feasible to use the stage motion to
create a smooth continuous transition between nanofibers of different
thickness for example, by gradually adjusting stage
acceleration/deceleration.

[0051] Electrospinning onto 3D Structures

[0052] In another embodiment of the invention, a method may be applied to
integrate low-voltage NFES "writing" capability with three-dimensional
("3D") substrates by suspending nanofibers on carbon micropost arrays
located on a Si substrate. In an example of this "writing" capability,
posts having a height of 40 μm, a diameter of 30 μm, an interpostal
distance of 100 μm were used. These carbon post arrays are fabricated
by the pyrolysis of high-aspect ratio SU-8 structures in a reducing
environment. See e.g., Kudryashov et al., "Grey scale structures
formation in SU-8 with ebeam and UV," Microelectron Eng. 67, 306-311
(2003); Malladi et al., "Fabrication of suspended carbon microstructures
by e-beam writer and pyrolysis," Carbon, 44, 2602-2607 (2006); Wang et
al., "A novel method for the fabrication of high aspect ratio CMEMS
structures," J. Microelectromech Syst. 14, 348-358 (2005).

[0053] The writing of suspended polymeric nanofibers between carbon posts
in an array was successfully carried out at a voltage of 200V. In this
experiment, nanofiber deposition was monitored in situ through a stereo
microscope. SEM images in FIG. 7 show that both individual and multiple
nanofibers 801 were directly suspended between the posts 802. These
nanofibers 801 can be coated with metal to function as connectors and
sensing elements on 3D microstructures. In the latter case, the sensing
elements will exhibit higher signal-to-noise ratio compared to flat
electrode geometries, resulting in enhanced sensitivity for chemical and
biological sensors. Pyrolysis of these polymeric nanofibers into carbon
will also enable conductive behavior with additional shrinkage of
dimension and versatile functionalization chemistry.

[0054] Formulations of the Polymer Solution

[0055] For continuous electrospinning operations, the optimum polymer mix
is observed to be within the range of about 20% to about 30% PEDOT:PSS
dispersion concentration in 1.6-2.0% PEO base solution where the % refers
to the wt/v %. For example 2% PEO refers to 2 g of PEO in 100 ml of
solvent (e.g., water). This formulation can be electrospun under
different humidity conditions ranging from about 40% to 80% relative
humidity. The nozzle-to-substrate distance varies in the range of about
1.0 to about 1.5 mm. The nanofibers are electrospun continuously with a
stable polymer jet. FIG. 8 depicts a model for the distribution of
PEDOT:PSS in the PEO bulk polymer specifically a set of approximately one
hundred (100) nanofibers 901 laid down for testing into a parallel array
on the gold electrodes 902. The inset of FIG. 8 shows the arrangement of
PEDOT:PSS islands in PEO solution. The nanofibers are conductive when the
PEDOT:PSS islands are in contact with each other

[0056] Several polymer blends, different humidity levels and various
nozzle-to-substrate distances were tested to achieve a stable nanofiber
jet that was then used to lay down an array of one hundred (100) parallel
conducting nanofibers between two gold pads separated 0.5 mm apart as
shown in FIG. 9A. This topology allowed for easy measurement of the
conductivity of the resulting nanofibers. An optimal balance of
viscosity, elasticity and conductivity was established to ensure
continuous nanofibers. At high concentrations of PEDOT: PSS (e.g., 30-60%
w/v PEDOT in 1.8% to 2% w/v PEO) led to spraying of short vertical fibers
that dried before reaching the substrate while too low PEDOT: PSS
concentrations (e.g., 0-20% w/v PEDOT in 1.8% to 2% w/v PEO) did not
produce conductive nanofibers. As expected, a higher deposition voltage
(600V) produced thicker nanofibers in the range of 1 μm as shown in
FIG. 9B while a lower deposition voltage (400V) produced thinner
nanofibers with diameters in the range of 200 nm as also shown in FIG.
9C.

[0057] At lower concentrations of PEDOT:PSS dispersion in PEO, the
dispersion forms PEDOT:PSS polymer islands in the PEO bulk solution. This
impedes the conductivity of the mixture since the PEDOT:PSS polymer
chains have to be in contact to conduct electricity effectively.
Moreover, the distribution of the islands is highly random and the
electrospun nanofibers obtained with PEDOT:PSS concentration below 20%
are usually non-conductive. Conversely, a very high concentration of
PEDOT:PSS (>30%) in PEO results in a highly conducting solution. This
formulation also has lower viscoelasticity due the short PEDOT:PSS
polymer chains, which interfere with the entanglement of the long PEO
polymer chains thereby reducing elasticity. Thus, the near field
electrospinning of this formulation generally leads to multiple short
vertical microfibers instead of continuous electrospinning of individual
nanofibers. The vertical fibers dry before reaching the substrate,
producing an array of standing microfibers.

[0058] PEDOT:PSS exist as a particle dispersion in water that, upon mixing
with PEO, is re-distributed as individual islands in the PEO matrix. This
generally restricts the conductivity of PEDOT:PSS in PEO. However, at a
critical concentration the individual islands start forming contacts with
each other yielding a conductive pathway. This critical concentration is
found to be at around 20% PEDOT:PSS in PEO.

[0059] As explained herein, electrospinning is initiated at an initial,
high voltage (e.g., a voltage above 600V). Once a polymer jet is induced,
the voltage is then reduced to thin down the jet of ink for the
production of nanofibers. A direct correlation is observed between the
voltage and thickness (or diameter) of the nanofibers as previously
described herein. The deposition of the nanofibers is carried out on a Si
wafer coated with a 500 nm thick insulating SiO2 layer. In the
experimental setup, the nanofibers were laid down between two gold
electrode strips 2 mm wide, separated by 1 mm gap. The current-voltage
(I-V) characteristics of the nanofibers was then measured between these
gold electrodes using a high precision Potentiostat in two electrode
voltammetric mode. In addition, the conductivity of the nanofiber arrays
between the gold pads was measured with a multimeter. The resistance was
found to be in the range of few hundred kΩs for thicker fibers and
few MΩs for the thinner fibers.

[0060] The current-voltage (I-V) response of thinner nanofibers deposited
at 400V is seen in the graph illustrated in FIG. 10. The two curves
illustrate variations in the output current since the nanofibers were
scanned multiple times. The variations are due to hysteresis caused by
thermal noise in the polymer chain. The I-V response in FIG. 10 is very
similar to a Schottky-diode characteristic. The Schottky-diode
characteristic indicates the formation of a Schottky-barrier formed
between the nanofiber and the gold electrode contact. This
Schottky-barrier is believed to be caused by the lower PEDOT content in
the nanofibers. This can be better understood as resulting from a smaller
number of conducting PEDOT islands on the nanofiber surface, leading to a
non-ohmic electrical contact. Similar formation of Schottky barriers have
also been reported by Hongzhi et. al. and Wang et. al. in carbon
nanotubes laid down on metal electrodes and used for making infrared
sensors See C. Hongzhi et al., "Development of Infrared Detectors Using
Single Carbon-Nanotube-Based Field-Effect Transistors," Nanotechnology,
IEEE Transactions on, vol. 9, no. 5, pp. 582-589 (2010); Wang et al. "A
novel method for the fabrication of high-aspect ratio C-MEMS structures,"
Microelectromechanical Systems, Journal of, vol. 14, no. 2, pp. 348-358
(2005).

[0061] The I-V response of thicker nanofibers deposited at 600V is shown
in FIG. 11. This response is largely ohmic, unlike the thinner fibers,
indicating the formation of a better electrical contact between the
nanofiber and the electrode. This technique can be used in continuous
writing of conducting nanofibers on flexible substrates to form simple
circuit elements that can be utilized for building fully integrated
polymer devices. LV-NFES offers complimentary and enhanced capability to
conventional printing technologies for conducting polymers due to the
wide range of dimensions that can be produced using LV-NFES on a single
substrate using a single printing technique.

[0062] While the addition of SU8 to high molecular weight PEO permits the
ink formulation to be pyrolysed, the use of SU8 directly as an ink for
NFES does not allow continuous electrospinning due to the limited
viscoelasticity of SU8. To address this problem, blending of high
molecular weight PEO with SU8 attributes the mixture, the viscoelastic
properties of PEO and the carbonization properties of SU8. In this
regard, mixture of PEO to SU8 in gamma-Butyrolactone (GBL) as a solvent
is employed for electrospinning on the NFES setup. The resulting mixture
is electrospun in different ratios leading to the generation of
micro/nanofibers as shown in FIG. 12.

[0063] The resulting mixture was easily electrospun in different ratios
but lead to the generation of thicker fibers. A 50:50 ratio of
SU8:HMW-PEO is found to achieve the right balance of solvent evaporation
induced hardening and stretchability resulting in continuous
electrospinning A higher ratio of SU8 led to the drying of the
electrospinning jet. A stepper motor stage was programmed to move the Si
substrate in a zig-zag square wave pattern.

[0064] The fibers as shown in FIG. 12 are pyrolysed at 900° C.
under an N2 gas flow rate of 2500 sccm throughout the process. It is
seen that the PEO:SU8 blend fibers were carbonized into carbon fibers
after pyrolysis. Significant porosity is observed in the fibers as seen
in the SEM pictures in FIGS. 13A and 13B represented by the darker porous
regions of the fiber. The two carbonized fibers shown in FIGS. 13A and
13B are obtained at different stage speeds. The diameter of the fibers is
found to shrink by approximately 40% after pyrolysis as seen in the date
illustrated in FIG. 14.

[0065] While embodiments have been shown and described, various
modifications may be made without departing from the scope of the
inventive concepts disclosed herein. The invention(s), therefore, should
not be limited, except to the following claims, and their equivalents.